A Refined Picture of the YBa2Cu3Ox Structure: Sequence of Dimpling-Chain Superstructures, 1D-Modulation of the Planes, Phase Separation Phenomena

Abstract

High resolution volumetric oxygen determination of over hundred Y-1230x equilibrium samples allowed to find different properties with mesoscopic and macroscopic methods. Thus, whereas X-ray and neutron high resolution structure refinements (lenght scale ≅ 500A) do show homogeneous samples, Raman scattering (length scale ≅ 40A) shows an intrinsic inhomogeneity of Y-123Ox. In the underdoped region a sequence of superstructures exists with stepwise softening of the O2/O3 in-phase mode (Alg) corresponding to decreasing dimpling (Cu-O2/O3 distance) with increasing oxygen doping. These superstructures have oxygen contents corresponding to the 2a0, 3a0, 5a0......oxygen vacancies chain-superstructures, found in the past with electron microscopy. This relationship between the 1D-vacancies ordering of the chains and the distance between the oxygens (O2,O3) and the copper (Cu2) of the planes, implies a 1-D modulation of the planes along the c-axis, possibly via the apical bond. At the onset of the overdoped range neutron diffraction shows the existence of a phase transition and EXAFS illustrate its mechanism (displacive transformation). In the overdoped phase Raman scattering supports the existence of two phases, as shown in the past from the splitting of the diamagnetic and resistive transitions, as well as of the peak of the specific heat.